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MG Midget and Sprite Technical - Flat battery

came to use the car today with the sun being out and discovered my battery was dead flat, which surprised me as i had left it connected to my ctek battery maintainer but when i looked it was plunged in but the socket was turned off. Recharged battery from my other charger and all was fine can a maintainer discharge a battery if it is disconnected from the mains supply?
mark heyworth

More likely a faulty battery or parasitic drain somewhere in the vehicle.

Charge up again, and see what happens if left for a while with the Ctek disconnected from the car.

Use a clamp meter on the earth cable from the battery to check for a parasitic drain. If you have one, then it is a matter of pulling fuses or disconnecting components to find the culprit (often audio equipment or anything with a circuit board)
Tim C

Mark
Highly unlikely the battery could feed current to a switched off but otherwise correctly functioning modern maintainer.
Bill B

As Tim says, audio equipment can often do this. Especially a modern stereo with Bluetooth connectivity.
The stereo can be sitting there unobtrusively searching for Bluetooth friends to connect with.
Also, assuming you're talking about a Spridget, the starter motor is a likely culprit for parasitic draw.

Unfortunately, a battery that has been drained dead flat is highly likely to be damaged.
Greybeard

A Ctek battery monitor definitely does draw current from the battery if the power is off---known as back draw-----it's not much but after a month or two sitting without a power supply the battery will be flat.
William Revit

just measured parasitic drain on battery and it is 0.047a so i think that will be the stereo memory as nothing else is on so starting to believe it is the ctek not being turned on that drained it.
mark heyworth

Coincidentally, I charged my Ryobi battery last week and then switched off the charger without removing the battery. Today I did remove the battery, pushed the test button and it was flat.
Dave O'Neill 2

I stand corrected! Maybe 10mA might be drawn from the battery. For a healthy fully charged battery it would take months to completely flatten the battery but, if the battery is a bit tired, you'd likely have starting problems or worse after a couple of months, as Willy says. How long was it connected but switched off Mark?

I'm surprised the Ctek will likely do this as I assumed there'd be a diode to block reverse current. Seems like poor design.
Bill B

Re Ryobi battery discharging on the charger, this doesn't happen with a DeWalt battery/charger.

Target "OK" battery drain on a vehicle with regular use is (less than) 50mA (0.050amps)

My battery has been sat for four months over the winter (with stereo memory on). Still turned over the engine for a bit, so battery wasn't flat flat, then I gave it a good charge with the Ctek.
Tim C

I'm surprised that a battery optimiser would allow back draw. Every day is a skool day, thanks Willie!
I assumed that the rectifier in the device, which is in essence a group of diodes, would put a stop to that sort of malarkey.
I have a small (cheap) optimiser connected to my ride-on mower battery which has been completely effective for several years, but I never considered that it could deplete the battery if the mains power was off.
So... a question.
If the power is switched off at the wall socket where does the juice go when it's isolated by the switch?
Is it simply charging the transformer coil as if it were a capacitor and being dissipated as heat?
That's my brain-worm for today. 🤯

I should probably go out more. 😳
Greybeard

It goes from one terminal of the battery, through the charger, to the other terminal of the battery. Just like if you leave the ignition on.
This happens with most battery chargers I have found. Maybe it is because Willie and I are upside down.
Paul Walbran

Grey-got this off the 'net

An unplugged battery charger connected to a battery will generally drain it very slowly, taking weeks or even months to cause significant damage, rather than hours or days. While some, particularly high-power or older, chargers might drain a battery faster, most modern smart chargers have minimal reverse current draw, often negligible enough to be safe for short-term storage.

Average Draw: Most chargers draw only a few milliamps (mA) in standby, or roughly 0.24-0.72 Ah per day.
Timeframe: A car battery with roughly 50-80 Ah capacity could take weeks or over a month to fully drain.
William Revit

I thought this was well known. I've always removed one charge clamp from the batt' if the charger isn't switched on. I seem to recall reading it yonks ago, in a workshop manual for something or other.

And monitors must draw at least some current in order to work; so if they're not giving out, they're taking it in, from the battery. Only full isolation prevents it.

Modern electronics has very little drain capacity, and reverse current flows. But it is still a factor, even if 'small'.

As Bill says though, a fully charged healthy battery should be able to stand it for a good while longer than overnight or a few days/a week, -- you'd think.



anamnesis

roughly 0.24-0.72 Ah per day

That is a lot. There is a decimal point missing ?

(0.024 - 0.072 Ah per day) ?
Tim C

not sure at all it's just what was written or more the way it was written-----I don't think it's current draw ,more the diminishing capacity of the battery,
If it were a 60Ah battery losing say 0.24Ah capacity per day that should be 240 days to run it dead flat so----8 months
William Revit

Ah yes, I see, what is lost, not what is drawing.
Tim C

This has been intriguing me so I've just checked my cheap Lidl smart charger (Ultimate Speed model), power off. I checked and re-checked the connections and my DMM showed zero draw, even on its microamp range. So it must depend upon the specific circuit. Smart chargers are chip controlled - there isn't a direct connection to the PSU rectifier, unlike a traditional charger, because the current has to be varied "intelligently".

Google AI suggests a circuit which has a "charging" LED and a "fully charged" LED plus resistors across the charging output so it may be that some smart chargers have a small amount of leakage through such LEDs, causing the stated "back draw".
Bill B

Willie and Paul...
Thank you fellas.
Obvious when you think about it. One pole to the other.
Brain-worm averted.
I'll sleep tonight 👌😁
Greybeard

Wakey wakey Greybeard -there's more---lol
As per Bill, it's been gnawing at me-- I don't have a Ctek , my charger is a small Arlec trickle charger--so.
Connected it to the battery with the mains unplugged with a digital multimeter in the circuit set to measure Amps.
Every few seconds there was a flick to 1.2A- very short but a flick. So being the inquisitive little prick that i am I connected the scope up to it and yep, every 4.3 seconds it gave a full(battery voltage)12.3V spike, very short but a spike.
At 1.2A / 12v it would be like dabbing your inside light globe onto the battery every 4.3 seconds.
Now-if we left our inside light turned on it would run the battery close to flat overnight, we've all done that. -but pulsing like this I'd 'guess' around a week or so for this particular charger---before the batter drained

willy

Also, have been having a readup on Ctek from various sources and their unplugged battery drain is rated(by them) at less than 1Ahour / month --bit of a weird rating but I'm taking that as loss of battery capacity(guessing) I think they're saying that the battery's capacity will be deminished by less than 1Ah per month so a 60Ah battery should last at least 60 months before dead flat with the Ctek connected which aligns with Bills findings--BUT then they recommend not to leave the Ctek connected without being powered---why
William Revit

Factoring in self discharge rate of lead acid and it'll be far less https://en.wikipedia.org/wiki/Self-discharge . IIRC older batteries don't fare as well and I don't think they like being in a discharged state either.
David Billington

This thread was discussed between 08/04/2026 and 12/04/2026

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